[1] Field of the Invention
The present invention relates to a development apparatus, an image forming apparatus and the like that develop a latent image formed on the image carrier with toner.
[2] Related Art
Some development apparatuses of image forming apparatuses have, for example, a developer roller and a doctor blade that comes in contact with the surface of the developer roller to regulate the amount of toner held thereon. These development apparatuses frictionally charge the toner by the doctor blade or the like to hold the frictionally charged toner on the surface of the developer roller, and develop a latent image on a photoreceptor drum with the toner. Such a development apparatus includes not only toner having a normal charging characteristic (normally charged toner), but also a considerable amount of toner whose charging characteristic has been degraded by abrasion due to friction or the like (degraded toner). This degraded toner includes toner charged with a polarity opposite to the normal polarity (reversely charged toner). Compared to the normally charged toner, the degraded toner is less likely to serve for development, and accordingly tends to remain in the development apparatus. An increase in the amount of the degraded toner remaining in the development apparatus poses a problem for the development, leading to degradation in image quality.
Regarding this problem, Japanese Laid-Open Patent Application No. 2004-170651 discloses a technology for compulsively consuming the degraded toner. Specifically, for instance, when a negative (minus) polarity is the charging polarity of the normally charged toner, a direct-current bias voltage (e.g. −300 V) is applied to the developer roller and at the same time, a direct-current bias voltage (e.g. −500 V) is also applied to the doctor blade. Herewith, the normally charged toner is attracted to the surface of the developer roller while the reversely charged toner is attracted to the doctor blade.
In this state of things, when the developer roller rotates, a layer of the normally charged toner is formed on the surface of the developer roller by electrostatic forces, and a part of the reversely charged toner attracted to the doctor blade adheres on top of the normally charged toner layer to thereby form a layer of the reversely charged toner. By development, the reversely charged toner on the developer roller is moved to the photoreceptor drum, and then cleaned therefrom by a cleaner.
This technology, however, still leaves a problem. That is, since the reversely charged toner is attracted to the doctor blade by electrostatic forces, only a part of the reversely charged toner is actually discharged from the developer roller via the photoreceptor drum, and a lot of reversely charged toner is left behind in the development apparatus.